1. Field
The present invention generally relates to the field of wireless communication systems. More specifically, the invention relates to transmission for wideband code division multiple access communication systems using multiple input multiple output channels.
2. Background
In wireless communication systems several users share a common communication channel. To avoid conflicts arising from several users transmitting information over the communication channel at the same time, some regulation on allocating the available channel capacity to the users is required. Regulation of user access to the communication channel is achieved by various forms of multiple access protocols. One form of protocol is known as code division multiple access (CDMA). In addition to providing multiple access allocation to a channel of limited capacity, a protocol can serve other functions, for example, providing isolation of users from each other, i.e. limiting interference between users, and providing security by making interception and decoding difficult for a non-intended receiver, also referred to as low probability of intercept.
In CDMA systems each signal is separated from those of other users by coding the signal. Each user uniquely encodes its information signal into a transmission signal. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the information. The encoding of the information signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the information signal. For this reason CDMA is also referred to as “spread spectrum” modulation or coding. The energy of each user's signal is spread across the channel bandwidth so that each user's signal appears as noise to the other users. So long as the decoding process can achieve an adequate signal to noise ratio, i.e. separation of the desired user's signal from the “noise” of the other users' signals, the information in the signal can be recovered. Other factors which affect information recovery of the user's signal are different conditions in the environment for each subscriber, such as fading due to shadowing and multipath. Briefly, shadowing is interference caused by a physical object interrupting the signal transmission path between the transmitter and receiver, for example, a large building. Multipath is a signal distortion which occurs as a result of the signal traversing multiple paths of different lengths and arriving at the receiver at different times. Multipath is also referred to as “time dispersion” of the communication channel. Multipath fading may also vary with time. For example, in a communication unit being carried in a moving car, the amount of multipath fading can vary rapidly.
A number of methods have been implemented to provide effective coding and decoding of spread spectrum signals. The methods include error detection and correction codes, and convolutional codes. In wireless communications, especially in voice communications, it is desirable to provide communication between two users in both directions simultaneously, referred to as duplexing or full-duplexing. One method used to provide duplexing with CDMA is frequency division duplexing. In frequency division duplexing, one frequency band is used for communication from a base station to a mobile user, called the forward channel, and another frequency band is used for communication from the mobile user to the base station, called the reverse channel. A forward channel may also be referred to as a downlink channel, and a reverse channel may also be referred to as an uplink channel. Specific implementation of coding and modulation may differ between forward and reverse channels.
The information in the user's signal in the form of digital data is coded to protect it from errors. Errors may arise, for example, as a result of the effects of time-varying multipath fading, as discussed above. The coding protects the digital data from errors by introducing redundancy into the information signal. Codes used to detect errors are called error detection codes, and codes which are capable of detecting and correcting errors are called error correction codes. Two basic types of error detection and correction codes are block codes and convolutional codes.
Convolutional codes operate by mapping a continuous information sequence of bits from the digital information of the user's signal into a continuous encoded sequence of bits for transmission. By way of contrast, convolutional codes are different from block codes in that information sequences are not first grouped into distinct blocks and encoded. A convolutional code is generated by passing the information sequence through a shift register. The shift register contains, in general, N stages with k bits in each stage and n function generators. The information sequence is shifted through the N stages k bits at a time, and for each k bits of the information sequence the n function generators produce n bits of the encoded sequence. The rate of the code is defined as R=k/n, and is equal to the input rate of user information being coded divided by the output rate of coded information being transmitted. The number N is called the constraint length of the code; complexity—or computing cost—of the code increases exponentially with the constraint length. A convolutional code of constraint length 9 and code rate 3/4, for example, is used in some CDMA systems.
The highly structured nature of the mapping of the continuous information sequence of bits into continuous encoded sequence of bits enables the use of decoding algorithms for convolutional codes which are considerably different from those used for block codes. The coding performed by a particular convolutional code can be represented in various ways. For example, the coding may be represented by generator polynomials, logic tables, state diagrams, or trellis diagrams. If the coding is represented by a trellis diagram, for example, the particular trellis diagram representation will depend on the particular convolutional code being represented. The trellis diagram representation depends on the convolutional code in such a way that decoding of the encoded sequence can be performed if the trellis diagram representation is known.
For signal transmission, convolutional coding may be combined with modulation in a technique referred to as “trellis coded modulation.” Trellis coded modulation integrates the convolutional coding with signal modulation in such a way that the increased benefit of coding more than offsets the additional cost of modulating the more complex signal. One way to compare different methods of signal transmission is to compare the bandwidth efficiency. Bandwidth efficiency is typically measured by comparing the amount of information transmitted for a given bandwidth, referred to as the “normalized data rate,” to the SNR per bit. The maximum normalized data rate that can possibly be achieved for a given SNR per bit is the theoretical maximum capacity of the channel, referred to as the “Shannon capacity” of the channel. The more bandwidth efficient a method of signal transmission is, the more nearly it is able to use the full Shannon capacity of the channel. A channel with multiple transmit antennas and multiple receive antennas which uses all possible signal paths between each pair of transmit and receive antennas, referred to as a multiple input multiple output (“MIMO”) channel, is known to have a higher Shannon capacity under certain channel conditions than a similar channel which uses only one transmit-receive antenna pair.
For signal reception, the signal must be demodulated and decoded. There are many methods of decoding convolutional codes, also referred to as “detection.” One method of decoding convolutional codes that uses the trellis diagram representation is Viterbi decoding. In the trellis diagram, each path through the trellis corresponds to a possible encoded sequence from the convolutional coder and the original information sequence that generated the encoded sequence. The Viterbi algorithm uses the encoded sequence actually received to determine a value of a metric for some of the paths through the trellis and to eliminate other paths from consideration. Finally, the decoder chooses a path through the trellis with the most favorable value of the metric, and the corresponding information sequence is thereby decoded. Thus, the Viterbi decoder provides maximum likelihood detection, as known in the art.
As stated above, one of the objects of coding is to protect the information in the user's signal from errors caused by various phenomena, for example, multipath fading. Another collection of techniques which can be used to increase signal reliability is referred to as “diversity.” Simply stated, diversity exploits the random nature of radio propagation by supplying to the receiver several replicas of the same information signal transmitted over independently fading—i.e. highly uncorrelated—signal paths for communication. For example, if one radio signal path undergoes a deep fade, another independent path may have a strong signal. By having more than one path to select from the signal to noise ratio of the information signal can be improved. One implementation of diversity is the RAKE receiver, which employs several antennas at the receiver to provide a selection of different signal paths. A shortcoming of the RAKE receiver is that its effectiveness breaks down at high data rates. One means of counteracting the effects of time dispersion or multipath is the use of orthogonal frequency division multiplexing (“OFDM”) as known in the art. OFDM works well at high data rates and thus avoids the shortcoming of ineffectiveness at high data rates with the RAKE receiver.
A further collection of techniques which can be used to increase signal reliability is referred to as “power control.” Simply stated, power control adjusts the power of the signal at the transmitter while the signal is being transmitted in order to compensate for varying conditions in the communication channel, such as relative movement of different users and multipath fading. Power control relies on the transmission of information regarding the condition of the channel, or “channel state information” (CSI) from the receiving unit back to the transmitter. Thus, power control is a CSI technique. There are other CSI techniques which involve, for example, the use of separate “pilot signals” and “training periods” of signal transmission. Diversity techniques, on the other hand are non-CSI techniques, in that no separate transmission of channel condition information is required for their implementation. In general, non-CSI techniques can be simpler and less costly to implement because non-CSI techniques avoid the complexity of transmitting channel state information.
Moreover, non-CSI techniques have an advantage over CSI techniques in that non-CSI techniques avoid incurring the “overhead” of transmitting channel state information, i.e. non-user information, on the channel. To the extent that channel capacity, i.e. the Shannon capacity for a given SNR per bit, is used to transmit non-user information, i.e. CSI, less channel capacity is available for transmitting user information, and the effective bandwidth efficiency of the transmission is, therefore, reduced. A channel which is not stable or for which channel conditions do not change slowly enough can require transmitting channel state information at high data rates to keep up with changes in channel condition in order for the transmitter to be able to make effective use of the channel state information. Thus, non-CSI techniques can provide an advantage for mobile communications where channel conditions are subject to rapid change.
The advantages of increased channel capacities of MIMO channels have been used in conjunction with a number of CSI techniques. The use of non-CSI techniques such as coding, diversity, and OFDM can also be used to improve the error performance and “throughput,” i.e. the data rate of user information, for wireless communications. Thus, there is a need in the art for taking advantage of the increased capacity of MIMO channels by increasing the effective bandwidth efficiency of transmission in MIMO channels while avoiding the disadvantages of transmitting channel state information. There is also a need in the art to provide improvements in error performance, data rate, and capacity of wireless communications in MIMO channels by exploiting increased bandwidth efficiency.